In the developing embryo, the primary vascular network is established by in situ differentiation of meso-dermal cells in a process called vasculogenesis. After embryonic vasculogenesis however, it is believed that all subsequent generation of new blood vessels, in the embryo or in adults, is governed by the sprouting or splitting of new capillaries from the pre-existing vasculature in a process called angiogenesis (Pepper, M. et al., 1996. Enzyme Protein, 49:138-162; Risau, W., 1997. Nature, 386:671-674). Angiogenesis is not only involved in embryonic development and normal tissue growth and repair, it is also involved in the female reproductive cycle, establishment and maintenance of pregnancy, and in repair of wounds and fractures. In addition to angiogenesis that takes place in the normal individual, angiogenic events are involved in a number of pathological processes, notably tumor growth and metastasis, and other conditions in which blood vessel proliferation is increased, such as diabetic retinopathy, psoriasis and arthropathies. Angiogenesis is so important in the transition of a tumor from hyperplastic to neoplastic growth, that inhibition of angiogenesis has shown promise as a cancer therapy (Kim, K. et al., 1993. Nature, 362:841-844).
Tumor-induced angiogenesis is thought to depend on the production of pro-angiogenic growth factors by the tumor cells, which overcome other forces that tend to keep existing vessels quiescent and stable. The best characterized of these pro-angiogenic agents or growth factors is vascular endothelial growth factor (VEGF) (Cohen et al., FASEB J., 13: 9-22 (1999)). VEGF is produced naturally by a variety of cell types in response to hypoxia and some other stimuli. Many tumors also produce large amounts of VEGF, and/or induce nearby stromal cells to make VEGF (Fukumura et al., Cell, 94: 715-725 (1998)). VEGF, also referred to as VEGF-A, is synthesized as five different splice isoforms of 121, 145, 165, 189, and 206 amino acids. VEGF121 and VEGF165 are the main forms produced, particularly in tumors (see Cohen et al. 1999, supra). VEGF121 lacks a basic domain encoded by exons 6 and 7 of the VEGF gene and does not bind to heparin or extracellular matrix, unlike VEGF165. Each of the references cited in this paragraph is incorporated by reference in its entirety.
VEGF family members act primarily by binding to receptor tyrosine kinases. In general, receptor tyrosine kinases are glycoproteins having an extracellular domain capable of binding one or more specific growth factors, a transmembrane domain (usually an alpha helix), a juxtamembrane domain (where the receptor may be regulated, e.g., by phosphorylation), a tyrosine kinase domain (the catalytic component of the receptor), and a carboxy-terminal tail, which in many receptors is involved in recognition and binding of the substrates for the tyrosine kinase. There are three endothelial cell-specific receptor tyrosine kinases known to bind VEGF: VEGFR-1 (Flt-1), VEGFR-2 (KDR or Flk-1), and VEGFR-3 (Flt4). Flt-1 and KDR (also known as VEGFR-2 or Flk-1, which are used interchangeably herein) have been identified as the primary high affinity VEGF receptors. While Flt-1 has higher affinity for VEGF, KDR displays more abundant endothelial cell expression (Bikfalvi et al., J. Cell. Physiol., 149: 50-59 (1991)). Moreover, KDR is thought to dominate the angiogenic response and is therefore of greater therapeutic and diagnostic interest (see Cohen et al. 1999, supra). Expression of KDR is highly upregulated in angiogenic vessels, especially in tumors that induce a strong angiogenic response (Veikkola et al., Cancer Res., 60: 203-212 (2000)). The critical role of KDR in angiogenesis is highlighted by the complete lack of vascular development in homozygous KDR knockout mouse embryos (Folkman et al., Cancer Medicine, 5th Edition (B. C. Decker Inc.; Ontario, Canada, 2000) pp. 132-152).
KDR (kinase domain region) is made up of 1336 amino acids in its mature form. The glycosylated form of KDR migrates on an SDS-PAGE gel with an apparent molecular weight of about 205 kDa. KDR contains seven immunoglobulin-like domains in its extracellular domain, of which the first three are the most important in VEGF binding (Cohen et al. 1999, supra). VEGF itself is a homodimer capable of binding to two KDR molecules simultaneously. The result is that two KDR molecules become dimerized upon binding and autophosphorylate, becoming much more active. The increased kinase activity in turn initiates a signaling pathway that mediates the KDR-specific biological effects of VEGF.
Thus, not only is the VEGF binding activity of KDR in vivo critical to angiogenesis, but the ability to detect KDR upregulation on endothelial cells or to detect VEGF/KDR binding complexes would be extremely beneficial in detecting or monitoring angiogenesis with particular diagnostic applications such as detecting malignant tumor growth. It would also be beneficial in therapeutic applications such as targeting tumorcidal agents or angiogenesis inhibitors to a tumor site or targeting agonists of KDR, VEGF/KDR, or angiogenesis to a desired site.
It is well known that gas filled ultrasound contrast agents are exceptionally efficient ultrasound reflectors for echography. Such ultrasound contrast agents include, for example, gas-filled microvesicles such as gas-filled microbubbles and gas filled microballoons. Gas filled microbubbles are particularly preferred ultrasound contrast agents. (In this disclosure the term of “microbubble” specifically designates a gaseous bubble surrounded or stabilized by phospholipids). For instance injecting into the bloodstream of living bodies suspensions of air- or gas-filled microbubbles in a carrier liquid will strongly reinforce ultrasonic echography imaging, thus aiding in the visualization of internal anatomical structures. Imaging of vessels and internal organs can strongly help in medical diagnosis, for instance for the detection of neoplastic, cardiovascular and other diseases.
For both diagnostic and therapeutic purposes it would be particularly beneficial to incorporate into gas filled ultrasound contrast agents, targeting vector compositions which exhibit high binding affinity for a desired target (such as, for example, KDR or the VEGF/KDR complex). For example, targeting vector-phospholipid conjugates and particularly targeting peptide-phospholipid conjugates may be used to prepare targeted, gas filled ultrasound contrast agents. In addition, it would be particularly beneficial to have methods for large scale production of highly purified forms of such targeting vector-phospholipid conjugates. Such compositions and methods would allow for production of compositions for use in diagnostic or therapeutic applications such as, for example, precise targeting of reporter moieties, tumoricidal agents or angiogenesis inhibitors to the target site.